Alpha-olefin oligomers and their use as hydraulic fluids and synthetic lubricants (synlubes) are well known. U.S. Pat. No. 2,937,129 reports the oligomerization of C.sub.5-14 .alpha.-olefins using a dialkyl peroxide catalyst to make a synlube. U.S. Pat. No. 3,113,167 describes an .alpha.-olefin oligomer process using a titanium halide and an aluminum compound.
The preferred catalysts for making .alpha.-olefin oligomers are Friedel Crafts metal halides such as BF.sub.3, U.S. Pat. No. 3,149,178. Optimum properties are obtained starting with 1-decene although mixtures of .alpha.-olefins have been used, U.S. Pat. No. 3,330,883.
The preferred Friedel Crafts catalyst is BF.sub.3. Pure BF.sub.3 is not an effective oligomerization catalyst. A small amount of polar compound is necessary as a promoter. U.S. Pat. No. 3,382,291 describes the use of alcohol promoters such as decanol. Other reported promoters are modenite (hydrogen form), water, phosphoric acid, fatty acids (e.g. valeric acid), ketones, organic esters, ethers, polyhydric alcohols, silica gel and the like.
Synlubes are generally classified by their viscosity at 100.degree. C. The viscosities most often referred to are 4, 6 and 8 centistokes (cs). Oligomers made from .alpha.-olefins are mixtures of dimer, trimer, tetramer, pentamer and small amounts of higher oligomers. Dimers are too volatile for most uses and are usually removed by distillation together with any unreacted monomer. In the case of 1-decene, 4 cs fluids are mainly trimer with a small amount of tetramer. The 4 cs synlubes can be merely distilled from the oligomer mixture leaving a more viscous residue that can be used to make 6 and 8 cs fluids. If demand for 6 and 8 cs fluids becomes large, the above procedure can be impractical because it could leave a manufacturer with an excess amount of trimer. Therefore a need exists for a procedure for making 6 and 8 cs fluids directly without the need for distilling out excessive amounts of trimer.
Pratt, U.S. Pat. No. 4,587,368, describes a process that results in increased tetramers and pentamers. Pratt achieves this in a 2-stage process by first conducting a conventional batch oligomerization procedure using a BF.sub.3 -propanol-hexanol catalyst until the mixture contains only 3-5 weight percent monomer. Then, in the second stage, additional .alpha.-olefin is added over an extended period to consume part of the trimer formed in the first stage thus increasing the amount of tetramer.